In motor vehicles, various periodic disturbances can occur in the steering system. For example, variations in road surface friction and profile, asymmetric brake torque and differential acceleration of the front wheels can cause disturbances in the longitudinal tire forces, lateral tire forces, tire self-aligning moment and suspension deflection forces for the front wheels. These disturbances, which can manifest themselves as steering rack force disturbances, are relayed to the driver through the steering column, affecting the steering feel of the vehicle and in some cases vehicle stability. To meet customer expectations with regard to a specific steering feel it can be advantageous for vehicle steering systems to have knowledge of such disturbances and the ability to reject or suppress them.
Accordingly, vehicle steering systems are generally designed to meet a specific steering feel based upon a reference tire. Drivers, however, will generally change a vehicle's tires at least once throughout the life of a vehicle, replacing the original manufacture (OEM) tires with new and perhaps inferior tires, thus altering the steering feel, which can depend upon the actual tire used on the vehicle.
Conventional electric steering systems, such as, for example, electric power assisted steering (EPAS), which is designed to assist a driver with vehicle steering by applying an auxiliary torque to the steering system, have some limited capability of suppressing rack force disturbances through techniques, including, for example, adaptive assist gain and/or controlled damping. Such methods, however, can also compromise steering feel.
Known methods for decoupling steering rack force disturbances in electric steering, which typically involve filters or very general comparisons between a total rack force and a reference rack force, can be impractical, oversimplified, and/or inaccurate for many real-world disturbances.